1. Field of the Invention
The present invention relates to heat exchangers for use in automotive air conditioning refrigerant circuits, and more particularly, to heat exchangers having a reduced thickness, over which the surface temperature is more evenly distributed, during operation.
2. Description of the Related Art
In FIG. 1, a known laminated-type heat exchanger, referred to as a "drain cup," is depicted. A heat exchanger 120 is constructed from a tank 101, a plurality of heat transfer tubes 102, a plurality of fins 103, and sideplates 106 and 107. All of these components are fixed together by brazing. Heat transfer tubes 102 and fins 103 are layered alternatively, with the outermost of fins 103 being provided with sideplates 106 and 107, respectively. Each of heat transfer tubes 102 defines a U-shaped flow path for fluid. The two ends of the U-shaped path are connected to tanks 101a and 101b, respectively. Tank 101a is divided further into two sub-tanks 101c and 101d by a partition (not shown). An inlet pipe 104 is connected to tank 101c, and an outlet pipe 105 is connected to tank 101d.
In FIG. 2, a schematic diagram illustrates the flow path of a fluid, for example, a refrigerant, within heat exchanger 120 according to the prior art. This mode of flow is referred to as a 4-path flow. Each of heat transfer tubes 102 is constructed from two plates 9, as shown in FIG. 3. In plate 9, an interior U-shaped flow path is formed by a shallow recess 10. A plurality of projections 13 are provided to create turbulence in the fluid flowing within heat transfer tubes 102. When assembling heat exchanger 120, protrusions 11 and 12 are connected to tank 101. The two plates 9 are connected face to face to form one heat transfer tube 102.
In the field of automotive air conditioning systems, it is desirable to reduce the size and the thickness of heat exchangers. However, reducing the size of the heat exchanger, while retaining the structure of conventional laminated-type heat exchangers, results in the problem of increased pressure loss. Generally, pressure loss in a heat exchanger is proportional to the length of the flow path, and inversely proportional to the cross-sectional area of the flow path. In order to reduce the thickness of heat exchanger 120, it is necessary to decrease the width of heat transfers tubes 102. Decreasing the width of heat transfer tubes 102 requires decreasing the width d of flow path 10, indicated in FIG. 3. Because the cross-sectional area of flow path 10 is proportional to its width d, decreasing d directly results in an increase in the pressure loss of the heat exchanger.
One attempt to solve this problem is depicted in FIG. 4. A heat exchanger 130 is constructed from a plurality of heat transfer tubes 102, a plurality of fins 103, sideplates 106 and 107, and a tank 111. Heat transfer tubes 102 are in fluid communication with tanks 111a and 111b. An inlet pipe 112 is connected to tank 111a, and an outlet pipe 113 is connected to tank 111b. In FIG. 5, a schematic diagram illustrates the flow path of refrigerant within the heat exchanger 130. This mode of flow of refrigerant is referred to as a 2-path flow.
Compared to 4-path flow heat exchanger 120, 2-path heat flow exchanger 130 has improved pressure loss characteristics and reduced size. In determining that 2-path flow heat exchanger 130 is superior to 4-path flow heat exchanger 120, it is assumed that 4-path flow heat exchanger 120, as depicted in FIG. 1, and 2-path flow heat exchanger 130, as depicted in FIG. 4, have the same number and same size of heat transfer tubes 102. With reference to FIGS. 2 and 5, the length of the flow path from inlet pipe to outlet pipe of 4-path flow heat exchanger 120 is twice that of 2-path flow heat exchanger 130. Accordingly, the pressure loss experienced by 2-path flow heat exchanger 130 is one-half that of 4-path flow heat exchanger 120. Further, the number of heat transfer tubes 102 that are directly in communication with the inlet pipe in 2-path flow heat exchanger 130 is twice that of 4-path flow heat exchanger 120. The total cross-section of the flow path of 2-path flow heat exchanger 130 is twice that of 4-path flow heat exchanger 120. Consequently, the pressure loss experienced by 2-path flow heat exchanger 130 is further reduced. As a result, 2-path flow heat exchanger 130 has an advantage of one-fourth of the pressure loss experienced by 4-path flow heat exchanger 120. In other words, ignoring the entire surface area for heat exchange, it is possible to reduce the size of 2-path flow heat exchanger 130 to one-fourth that of 4-path flow heat exchanger 120, while achieving the same pressure loss.
Still, the 2-path flow heat exchanger 130, as depicted in FIG. 4, has other disadvantages. For example, uneven temperature distribution occurs on the surface of 2-path flow heat exchanger 130 when the refrigerant circuit is operated. With reference to FIG. 5, the farther the heat transfer tubes 102 are from inlet pipe 112, the more active heat transfer occurs, or inversely, the nearer the heat transfer tubes 102 are to inlet pipe 112, the less active heat transfer occurs. When heat exchanger 130 is an evaporator, heat transfer tubes 102 that are farthest from inlet pipe 112 attain the lowest temperatures, and inversely, the temperature of heat transfer tubes 102 that are nearest to inlet pipe 112 is less reduced. The temperature difference between these heat transfer tubes may be several degrees.
In FIG. 5, a schematic diagram illustrates the flow of a fluid, for example, a refrigerant, flowing within 2-path flow heat exchanger 130. The refrigerant enters through inlet pipe 112 and travels to tank 111a. Within tank 111a, the refrigerant is distributed to each of heat transfer tubes 102. Also, within tank 111a, the refrigerant component that is more liquid reaches the deepest portion of tank 111a, because it is heavier. The component that is more gaseous, however, does not reach that portion of tank 111a, because it is lighter. This occurs because the refrigerant component with more liquid has a larger mass, and the refrigerant component that is more gaseous has a smaller mass. The flow velocities of refrigerant in each of heat transfer tubes 102 are about equal. As a result, a gradient in mass-flow rate from heat transfer tubes 102, on the right-hand side of FIG. 5, to those tubes 102, on the left-hand side of FIG. 5, is created. In other words, an imbalance in the mass-flow rate in the heat transfer tubes, corresponding to the distance from the inlet pipe, occurs. In the left-most heat transfer tubes, the mass-flow rate is highest and the most active heat transfer occurs, causing the surface temperature of the heat exchange to be significantly reduced. In the right-most heat transfer tubes, however, the mass-flow rate is lowest and the least active heat transfer occurs, causing the surface temperature to be less significantly reduced. This phenomenon is well known in the field of heat exchangers.
In accordance with the foregoing description, to reduce the thickness of the laminated-type heat exchanger, it is possible to change the flow mode from a 4-path flow to a 2-path flow. However, as noted above, 2-path flow heat exchangers experience spacial imbalance of heat transfer that decreases the overall heat transfer performance of the heat exchanger.